A conventional multi core optical fiber has a cross section shown in FIG. 1, in which the cores 1 are arranged in parallel, and are each other with substantially equal distanced and covered with the cladding 2. To integrate more cores each having a certain diameter in a unit cross sectional area, usually, the thickness of the cladding is decreased. Since the decrease of the thickness of the cladding results in the decrease of the distance between the adjacent cores, there arise some disadvantages such as cross-talk in communication. Therefore, the distance between the adjacent cores should be at least twice the core diameter and the maximum number to be integrated in a unit cross sectional area is limited.
Since the outer surface of the conventional multi core optical fiber has depressions 3, applied external stress is concentrated on these depressions so that it is more easily damaged than a single core optical fiber. In addition, since the depressions increase the outer surface area of the fiber there is an increase in the probability of the fiber breaking, and thus a fiber having good mechanical strength is not produced.